The present invention relates generally to the monitoring of the temperature of semiconductor devices, and more particularly to a method and circuit for monitoring the junction temperature of semiconductor devices.
Solid state devices such as diodes and thyristors are now well known in electric power applications. It is also well known that much of the heat generated in a power semiconductor device originates at its current carrying junction or junctions. As a result, the junction attains the highest temperature in the semiconductor device, and is the point at which semiconductor failure usually occurs, should the temperature become excessive. The term "junction" is used in this specification in its broadest sense. Semiconductor devices vary in the number of individual junctions that conduct current from anode to cathode. For example, a diode may have a single junction while a thyristor normally has a number of junctions. In the description and claims to follow, the use of the expression junction is intended to embrace both single and multiple junctions in semiconductors devices.
The determination of junction temperature cannot be accomplished with a satisfactory degree of accuracy by measuring the temperature of the structure employed to remove heat from the semiconductor device. Owing to the slow thermal transfer characteristics of these structures, external temperature measurements do not accurately reflect junction temperature. In applications of power semiconductor devices, it is impractical to measure junction temperature directly; as a result, the protective devices used have taken an indirect approach. The oldest and best known of such protective devices is probably the fuse, the particular fuse used being designed to blow at a current value considered safe for the semiconductor. This is a somewhat "rough and ready" approach, an approach which tends to not utilize the full current carrying capabilities of the semiconductor and one which is not usually fast enough to protect the semiconductor against all transient conditions. Moreover, fuses introduce the problem of matching the thermal characteristics of the fuse with those of the semiconductor.
Solid state electronic circuits are also known for protecting power semiconductors against overtemperatures. As would be expected, these circuits can be made more sensitive and faster acting than fuses. Circuits of this type can be found in U.S. Pat. No. 3,622,849, "Thyristor Junction Temperature Monitor" issued Nov. 23, 1971 to F. W. Kelley and F. L. Steen and in U.S. patent application, Ser. No. 635,141, "Temperature Monitoring Of Semiconductors" by David R. Boothman and Everett C. Elgar, filed Nov. 25, 1975.